1. Field of the Invention
The present invention relates to a super-resolution optical recording medium which can reproduce information by irradiating reproduction light onto recording marks formed in a recording layer. The present invention relates in particular to a super-resolution optical recording medium from which small recording marks with the size of the resolution limit or less of the reproduction optical system can be reproduced, and a method for recording information on the super-resolution optical recording medium.
2. Description of the Related Art
In recent years, as described in, for example, Japanese Patent Laid-Open Publication No. 2003-6872, a super-resolution optical recording medium has been proposed, from which a recording mark train smaller than the diffraction limit of a reproduction optical system can be reproduced.
In the case of a conventional optical recording medium, in general, it is impossible to read a recording mark train the period of which is a certain recording mark train period or less, using a reproduction method that uses light. The length of this recording mark train period is referred to as a diffraction limit. In a reproduction optical system with a wavelength of λ and a numerical aperture of NA, the diffraction limit is represented as λ/NA/2. If the length of a recording mark section is equal to that of a blank area in one period, then the length of the recording mark is represented as λ/NA/4. The length of the recording mark is referred to as a resolution limit.
Thus, reducing the wavelength λ and/or increasing the numerical aperture NA reduces the resolution limit, and hence increases the recording density. However, the ability to further shorten the wavelength and increase the numerical aperture is becoming limited. The aforementioned super-resolution optical recording medium has adopted technologies for reproducing recording marks smaller than λ/NA/4 to further increase the recording density without shortening the wavelength λ and increasing the numerical aperture NA.
The above-described conventional optical recording medium includes a phase-change recording film, as described in, for example, Scanning Probe Microscope Observation of Recorded Marls in Phase Change Disks: Takashi Kikukawa and Hajime Utsunomiya, Microsc. Microanal., 7 (2001) 363-367. Therefore, the recording mark or the recording layer in the vicinity thereof is not deformed by the recording operation.